Laser with means for suppressing back-ground fluorescence in the output

ABSTRACT

A tunable laser in which the direction of the output is invariant with frequency and is free of background fluorescence. The resonator includes a grating to tune the laser, a reflector that forms an auxiliary resonator with the grating, and a beam splitter therebetween that couples out a portion of the light returning from the grating through an aperture toward the reflector. The auxiliary resonator may employ nonreciprocal optics to reduce losses.

United States Patent 1 Shah [ June 12, 1973 LASER WITH MEANS FORSUPPRESSING BACK-GROUND FLUORESCENCE IN THE OUTPUT [75] Inventor:Jagdeep Chandravadan Shah,

Matawan, NJ.

[22] Filed: Apr. 3, 1972 [21] Appl. No.: 240,480

3,663,890 5/1972 Schulthess et a1. 331/945 Rrimgry Examinqr-William L.Sikes Attorney-W. L. Keefauver [57] ABSTRACT A tunable laser in whichthe direction of the output is invariant with frequency and is free ofbackground fluorescence. The resonator includes a grating to tune the[52] U5. Cl. 331/945 a a reflector that forms a auxiliary r s nat r with[51] Int. Cl. H015 3/08 h grating, n a beam splitter therebetween thatcou- [58] Field of Search 331/945; 350/160; pl Out a portion of thelight returning from the grating 250/ 199 through an aperture toward thereflector. The auxiliary resonator may employ nonreciprocal optics toreduce [5 6] References Cited losses.

UNITED STATES PATENTS 3,443,243 5/1969 Patel 331/945 4 Claims, 7 DrawingFigures TUNING MEANS 33 34 38 PUMPING g} IRIS LASER F U 5 49 I l 48 41GRATI N6 PAIENIED JUN I 2 I973 3 7 39 2 95 SHEET-1 0F 3 FIG./ w

DYE Z CELL TUNING MEANS l8 PuMPING I LASER I I II I2 I3 15 F/6.2 29PRIOR ART 9 I 23 PuMPING .M; DYE

LISISER LASEIZCELL POWER 25 OUT TUNING MEANS I8 F/G.3 w

, POWER PUMPING )/2e 37 our LASER as %24 35 TUNING MEANS PATENTEU3.739295 SHE BF 3 FIG. 4

TUNING MEANS 33 34 3% PUMPING g; IRIS LASER U so 49 I ll j r X 1 Q i 535 48 47 GRATING Fla 5 TUNING MEANS 33 34 3s g f F 35 W E GRATI NGGLAN-THOMPSON 46 PRgM PATENTEU 3973 S11E73 W 3 FIG. 6A

FLUORESCENCE DARK CURRENT FIG. 6B

PUMPING LASER a3 LASER WITH MEANS FOR SUPPRESSING BACK-GROUNDFLUORESCENCE IN THE OUTPUT BACKGROUND OF THE INVENTION This inventionrelates to means for supressing background fluorescence in the outputfrom a laser and, in particular, to a new coupling scheme for thatpurpose.

Dye lasers have been the subject of extensive recent research anddevelopment because of their extremely broad tuning ranges. Such a broadtuning range is of interest in proposed future optical communicationsystems. More immediately, it has proven useful for laboratoryapplications such as spectroscopy.

In the use of a dye laser beam as the light source, it is necessary toblock the background fluorescence of the laser from the sample underinvestigation. This condition would appear to be achievable by disposingthe sample at some significant distance from the output coupling pointof the laser and providing an aperture to block the fluorescence.Nevertheless, it has been found that continuous recording of data hasbeen difficult, or even impossible, in cases in which reduced backgroundfluorescence is of primary importance. The source of this difficulty istraceable to the preferred tuning technique for dye lasers whichtypically involves either the angular rotation of a diffraction gratingor the angular rotation of a prism about an axis normal to the directionof incidence of the laser light. The rotation of the tuning elementcauses a substantial deviation in the direction of the output beam.Thus, the beam may be substantially displaced from the point of intereston the sample under investigation; and the continuous recording of datais impaired unless the relative positions of some components of theapparatus are shifted to track the rotation of the tuning element.

It should be particularly noted that both continuous recording of dataand reduced background fluorescence are important conditions forstudying the excitation spectra of samples of new semiconductors. Theinvestigation of such semiconductors is important in a related branch ofthe laser art, namely, in the field of injection lasers. Reducedfluorescence background is also important in experiments such as Ramanscattering.

Further, in future optical communication systems, it is desirable thatany broadly tunable source emit its output beam in a direction that isinvariant with frequency.

Thus, for use of broadly tunable lasers, such as dye lasers, both inlaboratory equipment and for the use of such lasers in future opticalcommunication systems, it is desirable to provide output coupling thatsuppresses background fluorescence and permits the continuous recordingof data.

SUMMARY OF THE INVENTION I have discovered a simple new arrangement forextracting a tunable output from a broad spectrum laser, such as a dyelaser, and have found that this new arrangement solves the foregoingproblems. In a tunable laser according to my invention, the direction ofthe output is invariant with frequency and is substantially free ofbackground fluorescence.

According to a feature of my invention, the resonator includes arotatable tuning element to tune the laser, a reflector that forms anauxiliary resonator including the tuning element, the reflector beingbetween the tuning element and the active medium and a beam splitterdisposed between the tuning element and the reflector to couple out aportion of the light returning from the tuning element through anaperture toward the reflector.

According to a subsidiary feature of my invention, the auxiliaryresonator may employ nonreciprocal optics to reduce losses.

It is one advantage of my invention that it reduces the intensity of thebackground fluorescence of the broad spectrum laser, at least in thecase of a dye laser, to less than 1 X l0" of the laser intensity withina frequency band extending as much as 10 Angstroms from the laser lineand gives an output beam the direction of which is independent of thelaser wavelength. This new laser arrangement uses no external dispersingelements and permits a continuous recording of data as a function oflaser wavelength in the typical laboratory use of the laser.

It is an additional advantage of my invention that the same arrangementmay be used with gas lasers which have a series of relatively sharplaser lines which can be made to oscillate under appropriate tuningconditions. Simultaneous oscillation of such lines or insufficientlytunable selection of such lines often makes Raman spectroscopy orluminescence measurements of semiconductive crystals or other samplesvery difficult. Thus, my invention is applicable even to lasers such asargon ion lasers or helium-neon lasers, which are usually thought of asbeing narrow band lasers, and which have strong, sharp fluorescencelines in the close vicinity of the laser line. In other words, theinvention is applicable to any laser medium which has a broad spectrumof possible oscillations, or which has a significant tuning bandwidth.

BRIEF DESCRIPTION OF THE DRAWING Further features and advantages of myinvention will become apparent from the following detailed description,taken together with the drawing, in which:

FIGS. 1, 2 and 3 show differing partially pictorial and partially blockdiagrammatic illustrations of prior art arrangements of tunablebroadband lasers that exhibit the problems I have solved,

FIG. 4 shows a first partially pictorial and partially blockdiagrammatic illustration of a first embodiment of my invention;

FIG. 5 shows a modification of the embodiment of FIG. 4 to reducelosses; and

FIGS. 6A and 6B show curves which are useful in explaining the operationof my invention.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS Before proceeding to adescription of a detailed arrangement of my invention and its results, Ishall briefly discuss three configurations of typical prior-art-typeoptical resonators, two of which, shown in F [CS 1 and 2, have been usedheretofore to obtain useful tunable outputs from continuous-wave dyelasers.

In the prior art arrangement of FIG. 1, a pumping laser 11 is disposedoutside of the resonator of a dye laser and has its pumping beam focusedthrough the input mirror 13 by a lens 12. The focus of the beam isarranged to lie within the dye laser cell 14 through which the dyesolution is typically flowed to reduce damage effects. The opticalresonator is completed by a lens 15, which recollimates the highlyfocused pump and dye laser beams, a dispersive prism 16 and an outputmirror 17 which is typically rotated about an axis orthogonal to thepath of the incident beam and parallel to the apex edge of prism 16 bymeans of the tuning means 18 coupled to reflector 17. The prism 16establishes a slight angular and spatial displacement of differentpotential dye laser frequencies so that a particular one is selected foroscillation by the particular orientation of reflector 17.

In such a laser, the dye solution is typically rhodamine 6G in water,and the pumping laser 11 is typically an argon ion laser operating at488 nanometers or 514.5 nanometers.

As shown on FIG. 2, the optical cavity can be folded so that reflector23 replaces a lens as a focusing element. In addition, the pumping laserbeam can be introduced directly through a dispersive prism 16' in orderto reduce the threshold pumping level for oscillation by the use of ahighly reflective end mirror 17.

As shown in FIG. 3, a double-folded four-mirror cavity of the typedisclosed and claimed in the copending patent application of R.Rosenberg and P. K. Runge, Ser. No. 224,037, filed Feb. 7, 1972 andassigned to the assignee hereof, can also be used. For reasons describedin that application, the arrangement of FIG. 3 has a still lower pumpingthreshold for oscillation, since the astigmatism of reflectors 34 and 35compensates the astigmatism of cell 24 in a way that is compatible witha unique beam waist within cell 24.

In all of the arrangements of FIGS. 1-3 the output beam has either alarge fluorescence background amounting to about 1 X or l X 10' of thelaser intensity or a direction dependent upon the laser wavelength. Ifone attempts to solve this problem by using an external dispersingelement or a spectrometer to cut down the background, a continuousrecording of the data as a function of wavelength will still makedesirable the use of a set of complicated mechanical linkages that haveclear disadvantages. The mechanical linkages would become necessary inorder to track the position of the spectrometer with the output beam andthereby enable the continuous recording of data as a function ofwavelength.

In contrast, in the arrangement of FIG. 4 according to my invention,these problems are solved with a very simple arrangement. The basic ideaof the arrangement of FIG. 4 is to use an auxiliary resonator comprisinga mirror-grating combination 47 to tune the dye laser and to includewithin the mirror-grating combination 47 a beam splitter 50 to providean output port for the laser. Specifically, the mirror-gratingcombination 47 also includes the reflector 48 which has sufficientreflectivity that it is considered as completing the principalresonator, the rotatable grating 46 and tuning means 38 driving grating46. The latter components are similar to reflector 17 and tuning means18 of FIG. 1. Further, the auxiliary resonator includes the aperture 49inside the combination 47 to block fluorescence of the radiationreturning from the grating 46 toward the beam splitter 50. The apertureinside the cavity is not essential; it could be placed at the sample,for example.

There are two outputs from the beam splitter 50. That output which hasmost recently passed through aperture 49 is relatively free ofbackground fluorescence; but the component of the output which has mostrecently passed through reflector 48 still contains the typicalbackground fluorescence intensity, which remains about 1 X 10 to about IX 10 of the laser intensity for a frequency range of several dozennanometers of wavelength change from the laser line in either sense. Thefirst-mentioned output from beam splitter 50, propagating in the upwarddirection, has a background intensity reduced to about I X 10 to about iX 10 of the laser intensity within 10 Angstroms of the laser line. Thisresult is an improvement of several orders of magnitude.

The reason for this improvement is that the fluorescence is dispersed bygrating 46 and blocked by the iris or aperture 49 on the return pathfrom grating 46. The output direction remains invariant because it isdetermined by the direction of the axis needed for laser oscillation,which direction has also remained invariant. The characteristics of thelaser resonator compel the latter result. In other words, no componentof the laser resonator other than grating 46 has been shifted in itsorientation and there are no other dispersive elements in the resonator.

A further improvement in the performance is expected if the distancebetween grating 46 and the entrance cell of the spectrometer (not shown)that ultimately uses the first output is increased. This distance wasless than 1 meter from my initial experiments.

An obvious disadvantage of the arrangement of FIG. 4 is that the outputcoming from the direction of the laser active medium is being wasted.This disadvantage can be removed with a more sophisticated outputcoupling arrangement, as shown in the modified embodiment of FIG. 5.Here the beam splitter of FIG. 4 is re placed by a Glam-Thompsonpolarizer with the property that the light polarized parallel to theplane of the paper is transmitted but the light polarized orthogonal tothe plane of the paper is internally reflected at the diagonal interfacebetween the two component prisms of polarizer 60. The reflectedcomponent is coupled out through the side face of prism 16. Thetransmitted polarization is the dominant laser polarization of the dyelaser.

Also provided between grating 46 and aperture 49 is a phase shifter 58,which may be a quartz plate or a Babinet-Soleil compensator. Afterpassing through the phase shifter 58 from aperture 49, the beam becomeselliptically polarized. It remains so in small part even after it hasreturned from grating 46 through phase shifter 58, despite nearlycanceling phase shifts, because the grating 46 has differentreflectivities and introduces different phase shifts for the twoorthogonal polarizations, respectively. It will be noted that phaseshifter 58 is a reciprocal device; the nonreciprocal properties of theauxiliary resonator derive from the cooperation of phase shifter 58 andgrating 46. Thus, a certain fraction of the return beam will be coupledout by Glan-Thompson polarizer 60. The amount of the output coupling canbe varied by rotating the direction of the optic axis of the quartzplate, if such is used in phase shifter 58, or by an analogousadjustment. Indeed, the output coupling can be optimized in this way. Itwill be noted that all of the output power is monochromatic in the sensethat it has the reduced background fluorescence characteristic of myinvention. Also, the direction of the output beam is independent ofwavelength if the polarizer 60 is designed so that the output beam isorthogonal to the surface of the exit face thereof. The upper limit onthe efficiency of the arrangement of FIG. 5, as compared to the priorart embodiment of FIG. 3, is dependent upon the grating reflectivity;but it will usually be somewhat less because of feedback requirements ofthe laser.

The following are typical performance characteristics. Pumping therhodamine 6G dye solution with about 1 watt of multimode, multi-lineargon ion laser oscillations, the typical prior art output is about 50milliwatts without any tuning. With the tunable arrangement of my FIG.4, the undesired output including fluorescence, is about 25 milliwatts,whereas the useful output with reduced fluorescence is about milliwatts.If grating 46 has 1,800 lines per millimeter and is blazed at 500nanometers, the linewidth is about 0.02 nanometers to 0.03 nanometers;and the tuning range is about 50 nanometers.

l would expect the embodiment of FIG. 5 to give an increased clean poweroutput, as compared to that of FIG. 4, because of the lower losses.

The elimination of the background fluorescence can be appreciated inpart from the curves of FIGS. 6A and 6B in which the vertical orordinate axes are plotted to the same scale in units of a logarithm ofthe output intensity and the horizontal axis or abscissa showswavelength of the observed oscillating radiation. Curve 71, includingthe dye laser line 72 in FIG. 6A, is applicable to the prior artembodiments of FIGS. 1-3. Curves 81, 82 and 83 are applicable to theembodiment of FIG. 4. These spectra were obtained with a doublespectrometer and an electrometer. The only difference in plotting thespectra is that in FIG. 6B the abscissa is labeled in terms of units ofwavelength deviation from the dye laser line. It will be noted that inFIG. 6A the background fluorescence is substantial and is greater at agiven wavelength displacement from the laser line 72 than for the cleanoutput illustrated by curve 81 in FIG 6B for the embodiment of FIG. 4.For purposes of comparison, curve 82 shows the relative shape of thespectrum for the output still containing background fluorescence, thatis, the output deflected in the downward direction of FIG. 4. Curve 83shows the shape of the line of the argon ion pumping laser 11 in thesame wavelength region. The closeness of curves 81 and 83 indicates thedramatic degree of improvement of broadly tunable dye laser outputobtained in the embodiment of FIG. 4.

I claim:

1. An optical resonator comprising a principal resonator containingmeans including an active medium for generating both stimulated emissionof radiation and background fluorescence and an auxiliary resonatorcoupled to said principal resonator, said auxiliary resonator includinga tuning element that angularly disperses differing frequencies, saidprincipal resonator including a partially transmissive reflectordisposed between said tuning element and said active medium, and a beamsplitter between said tuning element and said partially transmissivereflector to provide an output port for said auxiliary resonator atleast for radiation propagating from said tuning element toward saidpartially transmissive reflector.

2. An optical resonator according to claim 1 in which the active mediumcomprises a dye laser medium having parallel major surfacessubstantially at Brewsters angle to the axis of said resonator forradiation at its laser wavelength to polarize linearly said radiationtransmitted therethrough, the tuning element is a dif fraction gratingand the beam splitter is a Glan- Thompson polarizer having itstransmission polarization direction aligned with the polarizationdirection of said major surfaces of said laser medium and in which theauxiliary resonator includes a phase shifter between the tuning elementand the beam splitter.

3. An optical resonator according to claim 2 in which the phase shifteris a reciprocal phase shifter that elliptically polarizes a linearlypolarized beam propagating from the direction of the Glan-Thompsonpolarizer.

4. An optical resonator according to claim 1 including an aperturebetween the tuning element and the beam splitter.

1. An optical resonator comprising a principal resonator containingmeans including an active medium for generating both stimulated emissionof radiation and background fluorescence and an auxiliary resonatorcoupled to said principal resonator, said auxiliary resonator includinga tuning element that angularly disperses differing frequencies, saidprincipal resonator including a partially transmissive reflectordisposed between said tuning element and said active medium, and a beamsplitter between said tuning element and said partially transmissivereflector to provide an output port for said auxiliary resonator atleast for radiation propagating from said tuning element toward saidpartially transmissive reflector.
 2. An optical resonator according toclaim 1 in which the active medium comprises a dye laser medium havingparallel major surfaces substantially at Brewster''s angle to the axisof said resonator for radiation at its laser wavelength to polarizelinearly said radiation transmitted therethrough, the tuning element isa diffraction grating and the beam splitter is a Glan-Thompson polarizerhaving its transmission polarization direction aligned with thepolarization direction of said major surfaces of said laser medium andin which the auxiliary resonator includes a phase shifter between thetuning element and the beam splitter.
 3. An optical resonator accordingto claim 2 in which the phase shifter is a reciprocal phase shifter thatelliptically polarizes a linearly polarized beam propagating from thedirection of the Glan-Thompson polarizer.
 4. An optical resonatoraccording to claim 1 including an aperture between the tuning elementand the beam splitter.